It is known that in a rolling bearing of which the outer race is rotated, since the flow of lubricating grease sealed in therein is restricted under centrifugal force, oil film is more difficult to form on required parts of the bearing compared to a rolling bearing of which the inner race is rotated.
The reason is explained with reference to FIG. 1. A rolling bearing comprises concentrically arranged inner and outer races 2 and 1, a plurality of rolling elements 3 disposed between the inner and outer races 2 and 1, a retaining 4 retaining the rolling elements in position, and seal members 5 closing openings at the respective axial ends. Grease G is sealed in the interior of the bearing closed by the seal members 5.
The portion of the grease G adhered to the surface of the stationary inner race 2 scarcely flows and thus scarcely serves to lubricate the bearing. The portion of the grease G adhered to the inner surface of the outer race 1 gets stuck to the outer race 1 under centrifugal force generated due to the rotation of the outer race 1 and is thus supplied to the inner portion of the bearing only at a low rate, except when it is moved by the retainer 4.
Thus, with this type of rolling bearing, once only a small amount of grease leaks out, lubricating oil is scarcely supplied to frictional surfaces of the rolling bearing, thus causing noise and destabilize torque due to poor lubrication.
In order to solve this problem, a grease-sealed rolling bearing of which the outer race is rotated is proposed in Patent document 1. With this bearing, lubricating grease having a dynamic viscosity at 40° C. of 30 to 70 cSt is sealed in the rolling bearing such that its content is 5 to 20% relative to the entire volume of the interior of the bearing.
But the rolling bearing disclosed in Patent document 1 is used to support a fan motor in a hard disk drive or an air-conditioner and is not intended for use in an outdoor cold environment. Also, this rolling bearing is not improved with respect to cold-time noise and its service life at high temperature.
Belt systems for automotive engines include timing belts for driving camshafts and engine accessory belts for driving engine accessories and other electric devices. Pulley units are used with these belt systems.
Such pulley units include tension pulleys for applying tension to belts, and idler pulleys for suitably guiding belts. According to their shapes, there are pulleys with a front groove for guiding the groove side of a belt, and pulleys with a flat back for guiding the flat side of a belt. Pulley units are rotatably supported by a grease-sealed rolling bearing including a rotary outer race. The lubricated grease sealed in the bearing contains base oil comprising ester oil or ether oil (Patent document 2).    Patent document 1: JP2001-123190A    Patent document 2: JP2005-060482A